Add `networks` TLV to `Init`'s TLV stream

Add `networks` TLV to `Init`'s TLV stream

This was a fairly old introduction to the spec to allow nodes to indicate
to their peers what chains they are interested in (i.e. will open channels
and gossip for).

We don't do any of the handling of this message in this commit and leave
that to the very next commit, so the behaviour is effectively the same
(ignore networks preference).